The present invention relates to mass spectrometry.
A mass spectrometer analyzes masses of sample particles, such as atoms and molecules, and typically includes an ion source, one or more mass analyzers and one or more detectors. In the ion source, the sample particles are ionized. The sample particles can be ionized with a variety of techniques that use, for example, chemical reactions, electrostatic forces, laser beams, electron beams or other particle beams. The ions are transported to one or more mass analyzers that separate the ions based on their mass-to-charge ratios. The separation can be temporal, e.g., in a time-of-flight analyzer, spatial e.g., in a magnetic sector analyzer, or in a frequency space, e.g., in ion cyclotron resonance (“ICR”) cells. The ions can also be separated according to their stability in a multipole ion trap or ion guide. The separated ions are detected by one or more detectors that provide data to construct a mass spectrum of the sample particles.
In the mass spectrometer, ions are guided, trapped or analyzed using magnetic fields or electric potentials, or a combination of magnetic fields and electric potentials. For example, magnetic fields are used in ICR cells, and multipole electric potentials are used in multipole traps such as three-dimensional (“3D”) quadrupole ion traps or two-dimensional (“2D”) quadrupole traps.
For example, linear 2D multipole traps can include multipole electrode assemblies, such as quadrupole, hexapole, octapole or greater electrode assemblies that include four, six, eight or more rod electrodes, respectively. The rod electrodes are arranged in the assembly about an axis to define a channel in which the ions are confined in radial directions by a 2D multipole potential that is generated by applying radio frequency (“RF”) voltages to the rod electrodes. The ions are traditionally confined axially, in the direction of the channel's axis, by DC biases applied to the rod electrodes or other electrodes such as plate lens electrodes in the trap. In a portion of the channel defined by the rod electrodes, the DC biases can generate electrostatic potentials that axially confine either positive ions or negative ions, but cannot simultaneously confine both. Additional AC voltages can be applied to the rod electrodes to excite, eject, or activate some of the trapped ions.
In MS/MS experiments, selected precursor ions (also called parent ions) are first isolated or selected, and next reacted or activated to induce fragmentation to produce product ions (also called daughter ions). Mass spectra of the product ions can be measured to determine structural components of the precursor ions. Typically, the precursor ions are fragmented by collision activated dissociation (“CAD”) in which the precursor ions are kinetically excited by electric fields in an ion trap that also includes a low pressure inert gas. The excited precursor ions collide with molecules of the inert gas and may fragment into product ions due to the collisions.
Product ions can also be produced by electron capture dissociation (“ECD”) or ion-ion interactions. In ECD, low energy electrons are captured by multiply charged positive precursor ions, which then may undergo fragmentation due to the electron capture. To induce ECD processes in ICR cells, the precursor ions and the electrons are radially confined by large magnetic fields, typically from about three to about nine Tesla. Axially, the positive precursor ions and the electrons are confined by electrostatic potentials in adjacent regions. Near the border of the adjacent regions, trajectories of the precursor ions and the electrons may overlap and ECD may take place. Alternatively, the trapped precursor ions may be exposed to a flux of low energy electrons.
Multipole ion traps typically use RF multipole potentials to radially confine ions. An electron's mass-to-charge ratio is one hundred thousand to one million times smaller than mass-to-charge ratios of typical precursor ions. Conventional multipole traps, however, can simultaneously confine only particles whose mass-to-charge ratios do not differ more than about a few hundred times. It has been suggested that ECD can be performed in a multipole trap if additional magnetic fields are used to trap the electrons or a large flux of electrons is introduced.
Ion-ion interactions have been used to generate product ions in 3D quadrupole traps, where an oscillating 3D quadrupole potential can simultaneously confine positive and negative ions in a central volume, and no electrostatic potentials are required to provide axial confinement.